1. Field of the Invention
The present invention relates to a printing apparatus and a printing method using multiple nozzle groups applicable to, for example, an inkjet serial- or line-printer, particularly to a printing apparatus equipped with a print head having multiple dot forming element groups arrayed at different group spacing from the dot forming element pitch and to a printing method therefor.
2. Description of the Related Art
Conventional printing apparatuses include, for example, the serial printer, which prints characters one by one, and the line printer, which prints a line of characters at one time. The serial nonimpact inkjet printer, for instance, provides a printout corresponding to the print data by driving a print head formed with multiple nozzles in the main scanning direction while jetting ink drops from the nozzles and conveying a printing paper or other printing medium in the sub-scanning direction perpendicular to the main scanning direction. Since this conventional inkjet printer forms adjacent dot lines on the printing medium with ink drops jetted from the same nozzles, however, the effect of variance in nozzle characteristics and the like is conspicuous and the print quality low.
This problem is addressed by, for example, U.S. Pat. No. 4,198,642, which teaches interlace printing with constant pitch sub-scanning, i.e., a printing scheme in which the number of used nozzles n and he nozzle pitch k are set to be relatively prime and paper feed is effected at a constant sub-scan amount of n dot pitch. Two integers are relatively prime when they do not have common divisors other than 1.
FIG. 1 is a diagram for explaining conventional interlace printing. The print head 100 has N nozzles #1-#9 arrayed in the sub-scanning direction at a prescribed nozzle pitch k.multidot.D (N=9 and k=4 in the illustrated example). Sub-scan feed of a printing paper is conducted at a constant feed amount L.multidot.D. In the example shown in FIG. 1, since all of the nozzles are used to jet ink drops, the number of nozzles N and the number of used nozzles n is the same. D denotes the printing resolution and is called the "dot pitch." Regarding the various parameters defined as integer multiples of the dot pitch D (k.multidot.D, L.multidot.D etc.) in the following description, only their integer portions are sometimes used. For instance, k may be called the "nozzle pitch" and L the "feed amount." When interlace printing is conducted, the nozzle pitch k and the sub-scan feed amount L (=n) are relatively prime. For example, if k=4 and the printing resolution in the sub-scanning direction is 360 dpi, the nozzle pitch k is 4 dots (4/360 inch). Similarly, the paper feed amount, i.e., the sub-scan feed amount L (=n) is 9 dots (9/360 inch).
As shown in FIG. 1, effecting a sub-scan of L dot pitch once every main scan of the print head 100 causes adjacent dot lines to be printed by different nozzles. For instance, the dot line formed by the first main scan pass of nozzle #7 is followed by a dot line formed by nozzle #5, which is followed by a dot line formed by nozzle #3, which is followed by a dot line formed by nozzle #1. Interlace printing can therefore produce high-quality printed images since it spreads out the effect of nozzle characteristic variance.
In the conventional inkjet printer of the interlace printing type, it is taken for granted that a constant nozzle pitch k can be secured. Based on this assumption, the nozzle pitch k and the number of driven nozzles n are set to be relatively prime and paper feed is conducted at a constant pitch of n dots.
Demand for higher printing speeds in recent years has heightened the need to form print heads with larger numbers of nozzles. Consistent formation of many nozzles at a constant nozzle pitch is, however, difficult. Nozzle pitch is apt to change midway and defects are likely to occur in some of the nozzles. When the prescribed nozzle pitch cannot be obtained, the quality of interlace printing by the prior art is markedly degraded owing to overwriting and/or skipping of raster lines. Since the prescribed nozzle pitch has to be secured in print heads, increase of the nozzle number will lower the production yield and raise the production cost proportionally. The prior art thus does not take into account the recent need for the larger number of nozzles. Since the prior art printing technique requires a constant nozzle pitch for all the nozzles of the nozzle head, it cannot be applied without modification to a printing apparatus having a large number of nozzles for which a constant nozzle pitch is difficult to obtain.